Vehicles recently include a communication system used for locking and unlocking a door of the vehicle through a remote controlling operation through an antenna mounted to the vehicle.
A conventional antenna for the above purpose will be explained.
FIG. 6 is a perspective view of the conventional antenna 100. A core 1 having a rectangular column shape is made of magnetic material of Ni ferrite and a coil 2 of metal wire, such as copper wire, is coated with heat-resistant resin and is wound a specific number of times on a predetermined region of the outer surface of the core 2. A screw 3 made of Ni ferrite magnetic material is inserted into a screw hole provided at an outside of the coil 2 on the core 1 as to move forward and backward by rotation of the screw. A wiring board 4 has patterns of wiring (not shown) provided on both, upper and lower, surfaces thereof. The wiring board 4 is mounted to the bottom of a recess 5A provided in case 5 made of heat-resistive resin. The wiring patterns are soldered to both ends of the coil 2. Electronic components 6, such as a capacitor 6A and a resistor 6B, are mounted on the wiring board 4 and electrically connected to the coil 2 by the wiring patterns, thus providing a series-resonant circuit.
The conventional antenna 100 having the foregoing arrangement may however have a varying resonant frequency since a capacitance of the capacitor 6A, a resistance of the resistor 6B, and an inductance of the coil 2 may vary.
When a coil moves close to magnetic material, a magnetic flux profile generally varies, thus changing the inductance accordingly.
According to the above principle, the screw 3 of magnetic material can move towards and from the coil 2 for changing the inductance of the coil 2, thus adjusting the resonant frequency of the antenna 100 to a desired frequency.
The series resonant circuit including the antenna 100 is electrically connected through the wiring patterns to an electronic circuit (not shown) of an internal communication device mounted to a door or a mirror of the vehicle, hence providing a communication system.
When a driver of the vehicle carrying a mobile card as an external communication device moves close to the vehicle or transmits radio waves from a mobile telephone as the external communication device, the antenna 100 receives a signal corresponding to the above operations. An identification code of the external communication device is then examined by the electronic circuit of the internal communication device for locking and unlocking the door.
An antenna having an adjustable resonant frequency is disclosed in Japanese Patent Laid-Open Publication No.10-341105.